Pressure control for partial stroke tests

ABSTRACT

A method of testing a valve actuator coupled to a process control device includes the steps of: controlling, with the process control device, a pressure within the valve actuator to ramp from an initial pressure towards a pre-determined pressure limit; monitoring a position of the valve actuator to detect a travel of the valve actuator; and upon one of the pressure within the valve actuator reaching the pre-determined pressure limit or detecting the travel of the valve actuator, controlling the pressure within the valve actuator to return to the initial pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/135,377, entitled “Pressure Control for Partial Stroke Tests” andfiled Mar. 19, 2015, the entire disclosure of which is herebyincorporated by reference herein.

FIELD OF THE DISCLOSURE

The present disclosure is directed to a method and apparatus forcalibrating and testing positioners and, more particularly, to a methodand apparatus for determining a bias of positioners and performingtests, such as partial stroke tests, with pressure control techniques.

BACKGROUND

In certain industries, such as the petroleum industry, partial stroketesting of emergency shutdown valves (ESVs) is increasingly required byregulatory bodies. However, ESVs and/or other valve assemblies that arepart of “Safety Instrumented Systems” (SISs) are generally designed foron/off operation and connections between valve stems and actuators arenot tight, resulting in significant lost motion. Further, ESVs aretypically characterized by high seal friction and prominent stick-slipdynamics. All of these factors contribute to poor throttling control andcomplicate partial stoke testing.

In addition, ESVs and/or other components of SISs are typically highgain devices. For example, SIS actuators are often single action pistonswith a spring return. A very small change in pressure within a chamberof an actuator can cause a large movement of the piston. As a result,when coupling SIS actuators, or other SIS components, to process controldevices (e.g., to perform partial stroke tests or other tests), biasesof the process control devices, such as I/P (current to pressure)biases, can have a dramatic impact on the calibration of the SIScomponents. If a calibration of the SIS components is off by asignificant amount, results from tests on the SIS components, such aspartial stroke tests, will be meaningless.

SUMMARY

In one embodiment, a method of testing a valve actuator coupled to aprocess control device comprises the steps of: controlling, with theprocess control device, a pressure within the valve actuator to rampfrom an initial pressure towards a pre-determined pressure limit;monitoring a position of the valve actuator to detect a travel of thevalve actuator; and upon one of the pressure within the valve actuatorreaching the pre-determined pressure limit or detecting the travel ofthe valve actuator, controlling the pressure within the valve actuatorto return to the initial pressure.

In further accordance with any one or more of the foregoing exemplaryaspects of the present invention, the method may further include, in anycombination, any one or more of the following preferred forms.

In one preferred form, the method further comprises the step ofdetermining a bias of the process control device controlling the valveactuator by comparing a measured pressure within the valve actuator to aset point pressure utilized by the process control device, whereincontrolling the pressure within the valve actuator to ramp from theinitial pressure towards a pre-determined pressure limit includesaccounting for the bias of the process control device.

In another preferred form, controlling the pressure within the valveactuator to return to the initial pressure includes: controlling thepressure to step from one of the pre-determined pressure limit or thepressure at which the travel of the valve actuator is detected to astepped pressure higher or lower than the pre-determined pressure limitor the pressure at which the travel of the valve actuator is detected;and after controlling the pressure to step, controlling the pressure toramp from the stepped pressure towards the initial pressure.

In another preferred form, monitoring the position of the valve actuatorto detect the travel of the valve actuator includes monitoring theposition of the valve actuator to detect a predetermined amount oftravel corresponding to a percentage of a possible travel of the valveactuator.

In another preferred form, controlling the pressure to ramp from theinitial pressure towards the pre-determined pressure limit includescontrolling the pressure based on a pressure control loop implemented bythe process control device, monitoring the position of the valveactuator to detect the certain amount of travel of the valve actuatorincludes monitoring the position of the valve actuator based on a travelcontrol loop implemented by the process control device, and the travelcontrol loop includes the pressure control loop.

In another preferred form, monitoring the position of the valve actuatorto detect the travel of the valve actuator includes monitoring theposition of the valve actuator to detect any non-zero travel of thevalve actuator.

In another preferred form, controlling the pressure to ramp from theinitial pressure towards the pre-determined pressure limit andmonitoring the position of the valve actuator includes controlling thepressure and monitoring the position of the valve actuator based on apressure control loop separate from any other control loops.

In another preferred form, the valve actuator is part of a safetyshutdown system.

In another preferred form, the valve actuator is coupled to a spoolvalve or a relay valve.

In another embodiment, a system comprises an actuator and a processcontrol device coupled to the actuator. The process control device isconfigured to control a pressure within the actuator to ramp from aninitial pressure towards a pre-determined pressure limit, gathermeasured values indicative of a position of the actuator to detect atravel of the actuator, and upon one of the pressure within the actuatorreaching the pre-determined pressure limit or detecting the travel ofthe actuator, control the pressure within the actuator to return to theinitial pressure.

In further accordance with any one or more of the foregoing exemplaryaspects of the present invention, the system may further include, in anycombination, any one or more of the following preferred forms.

In one preferred form, the process control device is further configuredto determine a bias of the process control device controlling the valveactuator by comparing a measured pressure within the valve actuator to aset point pressure utilized by the process control device, whereincontrolling the pressure within the valve actuator to ramp from theinitial pressure towards a pre-determined pressure limit includesaccounting for the bias of the process control device.

In another preferred form, determining the bias of the process controldevice includes: determining a calibration pressure value correspondingto a particular state of the valve actuator; controlling, with theprocess control device, the pressure within the valve actuator accordingto the set point pressure, wherein the set point pressure is based onthe calibration pressure value such that the valve actuator maintainsthe particular state; measuring an actual pressure within the valveactuator; and comparing the actual pressure to the set point pressure todetermine the bias of the process control device.

In another preferred form, controlling the pressure within the valveactuator to return to the initial pressure includes: controlling thepressure to step from one of the pre-determined pressure limit or thepressure at which the travel of the valve actuator is detected to astepped pressure higher or lower than the pre-determined pressure limitor the pressure at which the travel of the valve actuator is detected;and after controlling the pressure to step, controlling the pressure toramp from the stepped pressure towards the initial pressure.

In another preferred form, the process control device is furtherconfigured to determine the pre-determined pressure limit by driving thevalve actuator to a hard stop and measuring a pressure corresponding tothe hard stop.

In another preferred form, controlling the pressure within the valveactuator to ramp from the initial pressure towards the pre-determinedpressure limit includes controlling the pressure to: ramp the pressurefrom the initial pressure towards a pressure threshold according to afirst ramp rate and, upon reaching the pressure threshold, ramp thepressure from the pressure threshold to the pre-determined pressurelimit according to a second ramp rate different from the first ramprate.

In another preferred form, controlling the pressure within the valveactuator to ramp from the initial pressure towards the pre-determinedpressure limit includes controlling the pressure to ramp from theinitial pressure towards the pre-determined pressure limit according tothree or more ramp rates.

In another preferred form, controlling the pressure within the valveactuator to ramp from the initial pressure towards the pre-determinedpressure limit includes controlling the pressure to: ramp the pressurefrom the initial pressure towards a pressure threshold according to afirst ramp rate and, upon reaching the pressure threshold, ramp thepressure from the pressure threshold to the pre-determined pressurelimit according to a second ramp rate different from the first ramprate; wherein controlling the pressure within the valve actuator toreturn to the initial pressure includes: controlling the pressure tostep from one of the pre-determined pressure limit or the pressure atwhich the travel of the valve actuator is detected to a stepped pressurehigher or lower than the pre-determined pressure limit or the pressureat which the travel of the valve actuator is detected and, aftercontrolling the pressure to step, controlling the pressure to ramp fromthe stepped pressure towards the initial pressure.

In yet another embodiment, a computer device comprises one or moreprocessors and one or more non-transitory memories. The non-transitorymemories have computer executable instructions stored thereon that, whenexecuted by the one or more processors, cause the computer device to:control a pressure within an actuator to ramp from an initial pressuretowards a pre-determined pressure limit; gather measured valuesindicative of a position of the actuator to detect a travel of theactuator; and upon one of the pressure within the actuator reaching thepre-determined pressure limit or detecting the travel of the actuator,control the pressure within the actuator to return to the initialpressure.

In further accordance with any one or more of the foregoing exemplaryaspects of the present invention, the computer device may furtherinclude, in any combination, any one or more of the following preferredforms.

In one preferred form, controlling the pressure within the actuator toramp from the initial pressure towards the pre-determined pressure limitincludes controlling the pressure to ramp from the initial pressuretowards the pre-determined pressure limit according to two or more ramprates.

In another preferred form, controlling the pressure within the actuatorto return to the initial pressure includes: controlling the pressure tostep from one of the pre-determined pressure limit or the pressure atwhich the travel of the valve actuator is detected to a stepped pressurehigher or lower than the pre-determined pressure limit or the pressureat which the travel of the valve actuator is detected; and aftercontrolling the pressure to step, controlling the pressure to ramp fromthe stepped pressure towards the initial pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example process control system includingpositioners and/or actuator/valve assemblies that may be calibratedand/or tested with pressure control techniques;

FIGS. 2A, 2B, and 2C illustrate example stick slip dynamics of anemergency shutdown valve which can be implemented in the example processcontrol system illustrated in FIG. 1;

FIG. 3 illustrates an example positioner configured to calibrate and/ortest a actuator/valve assembly;

FIG. 4 illustrates an example flow diagram for determining a bias of apositioner, such as the positioner illustrated in FIG. 3;

FIG. 5 illustrates a block diagram of an example pressure control loopwhich can be, at least partially, implemented by the positionerillustrated in FIG. 3;

FIGS. 6A, 6B, and 6C illustrate various test result curves associatedwith a partial stroke test utilizing travel control;

FIGS. 7A, 7B, and 7C illustrate various test result curves associatedwith an example partial stroke test utilizing pressure controltechnique, which can be performed by the positioner illustrated in FIG.3;

FIG. 8 illustrates an example flow diagram for testing an actuator/valveassembly with pressure control techniques, which can be performed by thepositioner illustrated in FIG. 3;

FIGS. 9A, 9B, and 9C illustrate various test result curves associatedwith another example partial stroke test utilizing a stepped pressure,which can be performed by the positioner illustrated in FIG. 3;

FIGS. 10A, 10B, and 10C illustrate various test result curves associatedwith yet another example partial stroke test utilizing dynamic pressureramp rates, which can be performed by the positioner illustrated in FIG.3; and

FIG. 11 illustrates a block diagram of an example control loop includingan inner pressure control portion and an outer travel control portion,which can be, at least partially, implemented by the positionerillustrated in FIG. 3.

DETAILED DESCRIPTION

The present disclosure is directed to calibrating positioners or servocontrollers, such as valve positioners, using pressure controltechniques and utilizing the positioners or servo controllers to performtests with pressure control techniques. Specifically, the presentdisclosure is directed to a method and apparatus to: (i) determine abias of a positioner by controlling a pressure within, or supplied to,an actuator at constant volume, during end-point pressure control, at amiddle pressure value in a range of controlled pressures, or at anotherconvenient time or state of the actuator, and (ii) perform tests (e.g.,partial stroke tests) by controlling a pressure within or supplied to anactuator, rather than controlling a travel or position of the actuator.For ease of discussion, specific types of positioners, such as valvepositioners coupled to emergency shutdown valves, will be referred tothroughout this description. Generally, however, the method andapparatus of the present disclosure may be used to calibrate anysuitable components of control valve assemblies and utilize thosecomponents to perform tests with pressure control techniques.

By utilizing pressure control (i.e., as opposed to travel control) tocalibrate positioners and perform tests, the techniques of the presentdisclosure may alleviate certain difficulties resulting from the looseconnections, significant lost motion, high seal friction, andsignificant stick-slip dynamics that characterize many ESVs.Specifically, a valve positioner controlling an ESV may generate anestimate of current to pressure (I/P) bias for the valve positioner,which estimate of I/P bias is free of inconsistencies associated withlost motion and valve friction. The positioner may also perform partialstroke testing of an ESV while maintaining control of a pressure withinan actuator of the ESV, even in the event that the ESV is stuck.

The techniques of the present disclosure may also generally facilitatethe calibration and testing of any suitable positioners other thanpositioners coupled to ESVs, such as positioners coupled to andcontrolling compressor anti-surge valves, vent valves, etc. For example,a controller may cause a positioner coupled to a compressor anti-surgevalve to perform calibrations and tests using pressure controltechniques as described herein, where the compressor anti-surge valve isconfigured to prevent surges that occur when a compressor outletpressure is too high in relation to the flow through the compressor. Anexample controller may also cause a positioner coupled to a throttlingvalve/actuator assembly to perform calibrations and tests a usingpressure control techniques. By utilizing pressure control in thesescenarios, a controller may, for example, calibrate positioners whilethe positioners are in service, without disrupting a correspondingprocess.

Process Control System Overview

Referring now to FIG. 1, a process control system 10 constructed inaccordance with one version of the present disclosure is depictedincorporating one or more field devices 15, 16, 17, 18, 19, 20, 21, 22,and 71 in communication with a process control device, such as processcontroller 11. The process controller 11 may cause one or more of thefield devices 15, 16, 17, 18, 19, 20, 21, 22, and 71 to performcalibrations and/or tests using pressure control techniques, asdiscussed further below. The process controller 11 is also incommunication with a data historian 12 and one or more workstations 13,each having a display screen 14. So configured, the process controller11 delivers signals to and receives signals from the field devices 15,16, 17, 18, 19, 20, 21, 22, and 71 and the workstations 13 to controlthe process control system 10.

In additional detail, the process controller 11 of the process controlsystem 10 of the version depicted in FIG. 1 is connected via hardwiredcommunication connections to field devices 15, 16, 17, 18, 19, 20, 21,and 22 via input/output (I/O) cards 26 and 28. The data historian 12 maybe any desired type of data collection unit having any desired type ofmemory and any desired or known software, hardware or firmware forstoring data. Moreover, while the data historian 12 is illustrated as aseparate device in FIG. 1, it may instead or in addition be part of oneof the workstations 13 or another computer device, such as a server. Theprocess controller 11, which may be, by way of example, a DeltaV™controller sold by Emerson Process Management, is communicativelyconnected to the workstations 13 and to the data historian 12 via acommunication network 29 which may be, for example, an Ethernetconnection.

As mentioned, the process controller 11 is illustrated as beingcommunicatively connected to the field devices 15, 16, 17, 18, 19, 20,21, and 22 using a hardwired communication scheme which may include theuse of any desired hardware, software, and/or firmware to implementhardwired communications. The hardwired communications may include, forexample, standard 4-20 mA communications, and/or any communicationsusing any smart communication protocol such as the FOUNDATION® Fieldbuscommunication protocol, the HART® communication protocol, etc. The fielddevices 15, 16, 17, 18, 19, 20, 21, and 22 may be any types of devices,such as positioners, servo controllers, sensors, pressure regulators,control valve assemblies, etc., while the I/O cards 26 and 28 may be anytypes of I/O devices conforming to any desired communication orcontroller protocol. In the embodiment illustrated in FIG. 1, the fielddevices 15, 16, 17, and 18 are standard 4-20 mA devices that communicateover analog lines to the I/O card 26, while the digital field devices19, 20, 21, and 22 can be smart devices, such as HART® communicatingdevices and Fieldbus field devices, that communicate over a digital busto the I/O card 28 using Fieldbus protocol communications. Of course,the field devices 15, 16, 17, 18, 19, 20, 21, and 22 may conform to anyother desired standard(s) or protocols, including any standards orprotocols developed in the future.

In addition, the process control system 10 depicted in FIG. 1 includes anumber of wireless field devices 60 and 71 and a number of other fielddevices 61, 62, 63, and 64 communicatively connected to a wirelessrouter or other module 66. The field devices 60, 61, 62, 63, and 64 aredepicted as transmitters (e.g., process variable sensors) while thefield device 71 is depicted as a control valve assembly including, forexample, a control valve and an actuator. Wireless communications may beestablished between the process controller 11 and the field devices 60,61, 62, 63, 64, and 71 using any desired wireless communicationequipment, including hardware, software, firmware, or any combinationthereof now known or later developed. In the version illustrated in FIG.1, an antenna 65 is coupled to and is dedicated to perform wirelesscommunications for the field device 60 (e.g., a transmitter), while thewireless router or other module 66 having an antenna 67 is coupled tocollectively handle wireless communications for the field devices 61,62, 63, and 64 (e.g., transmitters). Likewise, an antenna 72 is coupledto the field device 71 (e.g., a control valve assembly) to performwireless communications for the field device 71. The field devices 60,61, 62, 63, and 64 or associated hardware such as module 66 and fielddevice 71 may implement protocol stack operations used by an appropriatewireless communication protocol to receive, decode, route, encode, andsend wireless signals via the antennas 65, 67, and 72 to implementwireless communications between the process controller 11 and the fielddevices 60, 61, 62, 63, 64, and 71.

The process controller 11 is coupled to one or more I/O devices 68 and70, each connected to a respective antenna 74 and 73, and these I/Odevices 68 and 70 and antennas 73 and 74 operate astransmitters/receivers to perform wireless communications with thewireless field devices 60, 61, 62, 63, 64, and 71 via one or morewireless communication networks. The wireless communications between thefield devices 60, 61, 62, 63, 64, and 71 (e.g., the transmitters and thecontrol valve assembly) may be performed using one or more knownwireless communication protocols, such as the WirelessHART® protocol,the Ember protocol, a WiFi protocol, an IEEE wireless standard, etc.Still further, the I/O devices 68 and 70 may implement protocol stackoperations used by these communication protocols to receive, decode,route, encode, and send wireless signals via the antennas 74 and 73 toimplement wireless communications between the process controller 11 andthe field devices 60, 61, 62, 63, 64, and 71.

As illustrated in FIG. 1, the process controller 11 conventionallyincludes a processor 23 that implements or oversees one or more processcontrol routines (or any module, block, or sub-routine thereof) storedin a memory 24. The process control routines stored in the memory 24 mayinclude or be associated with control loops being implemented within theprocess plant. Generally speaking, and as is generally known, theprocess controller 11 executes one or more control routines andcommunicates with the field devices 15, 16, 17, 18, 19, 20, 21, 22, 60,61, 62, 63, 64, and 71, the workstations 13 and the data historian 12 tocontrol a process in any desired manner(s).

Any one of the field devices 15, 16, 17, 18, 19, 20, 21, 22, and 71illustrated in FIG. 1, such as control valve assemblies or valvepositioners, and/or other suitable types of field devices utilized by aprocess plant, may be calibrated using pressure control techniquesand/or perform tests, such as Partial Stroke Tests (PSTs), with pressurecontrol techniques, as described herein. The process controller 11and/or a valve positioner coupled to the respective field device 15, 16,17, 18, 19, 20, 21, 22, and 71 may control a pressure supplied to anactuator and measure a pressure within the actuator to determine an I/Pbias and/or other suitable bias of the valve controller. The processcontroller 11 and/or a valve positioner may also ramp a pressure withinthe actuator up or down to a pressure limit to test a travel of theactuator (e.g., a travel of a piston of the actuator). FIG. 3illustrates an example of such a positioner and actuator in furtherdetail.

Positioners and Valve/Actuator Assemblies

In some implementations, one or more of the field devices 15, 16, 17,18, 19, 20, 21, 22, and 71 illustrated in FIG. 1, may be valvepositioners coupled to ESVs or other valve/actuator assembliesassociated with a SIS. In such cases, the ESVs or other valve/actuatorassemblies may primarily be on/off devices characterized by looseconnections, significant lost motion, high seal friction, and stick-slipdynamics. Examples of such characteristics are further illustrated inFIGS. 2A, 2B, and 2C.

Specifically, FIG. 2A is an example plot of relative travel vs. time fora one hundred second scan of a pneumatic valve actuator during which thetravel of the pneumatic valve actuator is controlled. As can be seen inFIG. 2A, the travel of the pneumatic valve actuator (bold line) is not asmooth curve or line. Rather, as a function of time, the travel of thepneumatic valve actuator includes various moments of sticking,represented by the flat line segments in the travel curve of FIG. 2A,followed by moments of slipping of the pneumatic valve, represented byvertical line segments or steps in the travel curve of FIG. 2A.

FIG. 2B illustrates a corresponding plot of actual pressure (within thepneumatic valve actuator) vs. time for the same one hundred second scanthat is described with reference to FIG. 2A. At each of the moments ofslipping of the pneumatic valve actuator, the pressure within theactuator dramatically shifts up or down due to a sudden change in volumewithin the actuator. These stick and slip dynamics are furtherillustrated in FIG. 2C, which includes a parametric plot of the travelsand pressures illustrated in FIGS. 2A and 2B (e.g., relative travel vs.actual pressure).

In some implementations, one or more of the field devices 15, 16, 17,18, 19, 20, 21, 22, and 71 illustrated in FIG. 1 may include positionersor servo controllers coupled to and controlling valve/actuatorassemblies other than ESVs. These other valve/actuator assemblies, suchas compressor anti-surge valves or vent valves, may primarily beconfigured for precision operations, such as throttling and control, incontrast to ESVs primarily configured as on/off devices.

Specifically, one or more of the field devices 15, 16, 17, 18, 19, 20,21, 22, and 71 illustrated in FIG. 1 may be positioners including spoolvalves. These spool valves included in the positioners may becharacterized by a balanced design allowing the spool valves to moveunder extreme conditions, such as very high pressures. As such,positioners including spool valves may operate similarly at manydifferent pressures, and positioners including spool valves may becalibrated and/or may perform tests at many different pressures orwithin a range of pressures.

In some cases, positioners including spool valves may utilize end-pointpressure control techniques. In particular, when a controlled valve isseated at an end-point (e.g., fully open or fully closed), this type ofpositioner may control a pressure within or supplied to the controlledvalve (“end-point pressure control”) such that the controlled pressureis below or above a maximum or minimum pressure, respectively, that canbe supplied to the controlled valve. In this manner, the positioner maymore quickly unseat or move the controlled valve from the end-point ascompared to scenarios in which the pressure is at the maximum or minimumpressure. Positioners described below may, in these cases, performcalibrations during end-point pressure control scenarios such that thepositioners and corresponding controlled valves are calibrated while thecontrolled valves are in service (e.g., without disrupting a process).

One or more of the field devices 15, 16, 17, 18, 19, 20, 21, 22, and 71illustrated in FIG. 1 may also be positioners including a pneumaticrelay, or poppet valve driven by a diaphragm assembly. This type ofpositioner may be characterized by an unbalanced design such that a biasof the positioner is dependent on the pressure supplied to or within acontrolled valve. Positioners including pneumatic relays may performcalibrations (e.g., of an I/P bias) at a middle pressure value within arange of pressure values, such as a range of pressure values defined bya bench set, in an implementation.

Turning now to FIG. 3, one potential example of a field device (such asfield devices 15, 16, 17, 18, 19, 20, 21, 22, and 71 illustrated inFIG. 1) is shown that includes an example positioner 200 that maycontrol an actuator/valve assembly 202, such as an actuator/valveassembly exhibiting behavior as illustrated in FIGS. 2A, 2B, and 2C oranother suitable actuator/valve assembly (a compressor anti-surge valve,a vent vale, etc.). In some cases, the positioner 200 may be configuredto perform partial stroke tests, or other tests, on the actuator/valveassembly 202. To this end, the positioner 200 may be pneumaticallyand/or electrically coupled to the actuator/valve assembly 202 via acoupling 204 and communicatively coupled to a controller, such asprocess controller 11 in FIG. 1.

In particular, the positioner 200 may control a pressure within orsupplied to the actuator/valve assembly 202 based on signals (e.g.,analog or digital) received from the controller and/or based on controllogic 208. For example, the controller may generate various signals(e.g., 4-20 mA signals) indicative of set point values or requests toperform calibrations and/or tests. Triggered by these signals from thecontroller, the positioner 200 may generate a pneumatic output tocontrol the actuator/valve assembly 202 based on the control logic 208stored on one or more non-transitory memories 210 of the positioner 200.The control logic 208 may implement at least a portion of one or morecontrol loops and may be executed by one or more processors 212 of thepositioner 200. The positioner 200 may, in some implementations,generate an internal current signal based on a current signal receivedfrom the controller (e.g., a 4-20 mA signal). This internal current maybe supplied to a current to pressure converter (I/P) and spoolvalve/relay component 230 of the positioner 200. Based on the internalcurrent signal, the I/P and spool valve/relay component 230 may generatethe output pressure supplied to the actuator/valve assembly 202 via thepneumatic coupling 204.

Control loops implemented by the control logic 208 may receive feedbackpressure and/or travel values from one or more sensors 214 in thepositioner 200 and/or any number of other sensors coupled to thepositioner 200 and/or to the actuator/valve assembly 202. These sensors214 may provide pressure values and/or travel values to the controllogic 208. Further details of example control loops that may, at leastpartially, be implemented by the example positioner 200 are describedwith reference to FIG. 11.

The control logic 208 may include a calibration routine 219. Whenexecuted by the one or more processors 212, the calibration routine 219may cause the positioner 200 to control a pressure in or supplied to theactuator/valve assembly 202. In some cases, the calibration routine 219may cause the positioner 200 to control a pressure in or supplied to theactuator/valve assembly 202 at constant volume, during end-pointpressure control, at a middle pressure value in a range of pressurevalue, or at any other suitable time or state of the actuator/valveassembly 202. In particular, the calibration routine 219 may operate inconjunction with the control logic 208 (as depicted in FIG. 3) or as astandalone routine to provide control signals to the I/P and spoolvalve/relay component 230. These signals may cause the I/P and spoolvalve/relay component 230 to control a pressure in or supplied to theactuator/valve assembly 202 while the calibration routine 219 adjusts anI/P bias or other suitable bias of the positioner 200. The calibrationroutine 219 may adjust the I/P bias by replacing a nominal bias with anadjusted bias such that the difference between the adjusted bias and thenominal bias is accounted for in future control of the actuator/valveassembly 202. Further details of an example method for calibrating apositioner with pressure control, which example method may be at leastpartially implemented by the calibration routine 219, are discussed withreference to FIG. 4.

In some implementations, the I/P and spool valve/relay component 230 ofthe positioner 200 may feedback a position of a spool valve and/orpneumatic relay of the I/P and spool valve/relay component 230 to thecontrol logic 208. The control logic 208 may utilize such a feedback ina damping term of a control loop, for example. To utilize this feedback,the calibration routine 219 may determine and/or adjust a null state ofthe spool valve or pneumatic relay, or a “minor loop feedback bias,” inaddition to or instead of an I/P bias. For example, a travel of apneumatic relay may be between 9,000 counts and 19,000 counts with anominal operating point of 13,000 counts. The feedback to the controllogic 208 in this example may be a normalized value dependent on themeasured travel (e.g., in counts) of the pneumatic relay minus thenominal operating point. Such a feedback signal is zero around a nullstate of the pneumatic relay and goes positive or negative depending onthe travel of the pneumatic relay, where the null state may be adjustedby the calibration routine 219.

The control logic 208 may also include a partial stroke test routine220. When executed by the one or more processors 212, the partial stroketest routine 220 may cause the actuator/valve assembly 202 to undergo apartial stroke test to test the operation of the actuator/valve assembly202. For example, the partial stroke test routine 220 may operate inconjunction with the control logic 208 (as depicted in FIG. 3) or as astandalone routine to provide control signals to the I/P and spoolvalve/relay component 230. These signals may cause the I/P and spoolvalve/relay component 230 to ramp a pressure in or supplied to theactuator/valve assembly 202 to cause a travel of the actuator/valveassembly 202. Further details of an example method for performing apartial stroke test, which example method may be at least partiallyimplemented by the partial stroke test routine 220, are discussed withreference to FIG. 8.

As discussed above, the controller may trigger or otherwise cause thepositioner 200 to initiate calibrations and/or to test (e.g., perform apartial stroke test), or the positioner 200 itself may initiate suchcalibrations or tests at periodic or otherwise determined times.Additionally, in some implementations, the positioner 200 or a separatedevice, module, or component operatively coupled to the positioner 200may include one or more buttons, switches, control panels, touchscreens,or other interfaces allowing a human operator to manually initiatecalibrations or test at the positioner 200 (e.g., by the pushing ofbuttons, entering of codes, etc.). In some cases, a human operator mayalso override previously initiated calibrations or partial stroke tests(e.g., initiated by the controller 206) so as to stop, cancel, orotherwise modify calibrations or partial stroke tests in certainsituations, such as emergency, testing, maintenance, or othersituations.

Although FIG. 3 illustrates the processors 212, memories 210, controllogic 208, calibration routine 219, and partial stroke test routine 220as components of the positioner 200, the controller may alternatively,or additionally, include at least some components substantially similarto the processors 212, memories 210, control logic 208, calibrationroutine 219, and partial stroke test routine 220. In fact, in someimplementations, the controller may implement all or most of thecalibration and testing functionality discussed with reference to FIGS.4 and 8 to control pressures within the actuator/valve assembly 202and/or to perform partial stroke tests on the actuator/valve assembly202. Generally, the functionality associated with controlling a pressurewithin the actuator/valve assembly 202 and/or performing partial stroketests on the actuator/valve assembly 202 may be distributed in anysuitable manner between the controller and the positioner 200.

Calibrating Positioners

FIG. 4 is a flow diagram of an example method 400 for calibrating apositioner, such as the positioner 200, using pressure controltechniques. Specifically, the example method 400 may be utilized todetermine a bias, such as a current to pressure (I/P) bias or minor loopfeedback bias, of the positioner 200. For ease of discussion, thecomponents of the example positioner 200 may be referenced in thedescription of the method 400, but, generally, the method 400 may beutilized to calibrate any suitable device coupled to an actuator/valveassembly and may be implemented by any suitable combination of acontroller and the device coupled to the actuator/valve assembly.

The positioner 200 may determine pressures corresponding to a particularstate of the actuator/valve assembly 202, such as one or more hard stopsor end-points, travel stops, stationary positions, middle points of arange of pressures (e.g., defined in a bench set), etc. (block 402). Insome cases, a bench set of an actuator may define a pressure range(e.g., three psig to fifteen psig) that corresponds to 0% to 100% travelof the actuator. In such cases, the positioner 200 may determine apressure just below a low end of the pressure range or just above a highend of the pressure range. For example, for a bench set of three psig tofifteen psig, the positioner 200 may determine a pressure between zeroand three psig to maintain a fixed volume at the low pressure end of thebench set of a pressure between fifteen and twenty psig to maintain afixed volume at the high end of the bench set. In other cases when abench set is not known, the positioner 200 may determine a pressurebased on pre-determined or approximated value. For example, thepositioner 200 may determine a pressure of 0+2=2 psig to maintain afixed volume at an estimated low end of actuator travel or a pressure of20−2=18 psig to maintain a fixed volume at an estimated high end ofactuator travel.

The positioner 200 may determine such pressure during a scenario inwhich end-point pressure control techniques are being utilized. Forexample, when the positioner 200 may perform end-point pressure controlto prevent a pressure within the actuator/valve assembly 202 fromreaching a maximum or minimum possible value of the pressure. Thepositioner 200 may determine a pressure value slightly below a maximumpressure value or slightly above a minimum pressure value while theactuator/valve assembly 202 is seated at an end-point (e.g., fully openor fully closed).

In still other cases, the positioner 200 may determine a pressure valueat near the middle or at another relative position within a range ofpressure values. For example, when the positioner 200 includes apneumatic relay, the positioner 200 may determine a particular pressuresomewhere in between pressure limits (e.g., defined by a bench set). Thedetermined pressure may be a pressure value in the middle of the range(e.g., having the same absolute value of pressure difference between themiddle value and both a high pressure limit and a low pressure limit).However, the positioner 200 may determine a pressure at any suitableposition in the range of pressures, such as a ten percent relativepressure, twenty percent relative pressure, etc. The positioner 200 mayeven determine multiple pressure values in a range of pressures so as todetermine multiple different bias values for a pneumatic relay.

Returning to FIG. 4, the positioner 200 may control a pressure in theactuator/valve assembly 202 while maintaining the actuator/valveassembly 202 at the determined particular state of the actuator/valveassembly 202 (block 404). That is, the positioner 200 may maintain aconstant volume within the actuator/valve assembly 202 while a pressurewithin the actuator/valve assembly 202 is controlled, maintain apressure within the actuator/valve assembly 202 near an end-point of theactuator/valve assembly 202 (e.g., during end-point pressure control),or maintain a pressure within the actuator/valve assembly 202 at aparticular pressure value in a range of pressure values. For example,the positioner 200 may control a pressure at a constant volume bycontrolling the pressure within the actuator/valve assembly 202according to a set point pressure value that is above or below thedetermined upper or lower pressure limit, respectively, determined atblock 402. Alternatively or additionally, the positioner 200 may controla pressure within a range of pressures or at a pressure value utilizedduring end-point control (e.g., while the actuator/valve assembly 202 isnear an end-point, such as fully open) by controlling the pressurewithin the actuator/valve assembly 202 according to a set point pressurevalue that is in the pressure range (e.g., in the middle of the range)or below/above a maximum or minimum pressure, respectively, anddetermined at block 402.

While the pressure is controlled at the particular state of theactuator/valve assembly 202, the positioner 200 may adjust a bias of thepositioner 200 based on the set point, a feedback of an actual pressurewithin the actuator/valve assembly 202, and/or a nominal or default biasof the positioner 200 (block 406). In some implementations, thepositioner 200 may adjust a default or nominal bias (e.g., existingvalues stored in the positioner 200 or default values provided to thepositioner 200) until a measure of error in the pressure over timesatisfies a convergence criterion (e.g., is at or below a threshold fora certain period of time). The measure of error may be at leastpartially based on a difference between the feedback of an actualpressure within the actuator/valve assembly 202 and the set point. Themeasure of error may, at least in some implementations, correspond to anintegral term in a proportional-integral-derivative (PID) controllerintegrated into the control logic 208.

The positioner 200 may update or integrate a bias of the positioner 200(e.g., the nominal or default bias) according to the adjustments made atblock 406 (block 408), or the positioner 200 may replace a default biaswith an adjusted bias based on the adjustments at block 406. Thisupdate/integration/replacement may ensure that subsequent control of theactuator/valve assembly 202 accounts for a most recently adjusted biasof the positioner 200. For example, before placing the positioner 200 inservice, an operator or the control logic 208 may configure thepositioner 200 with a default or nominal bias (e.g., by setting thetravel set point to 50% and turning on a travel integrator). Then, thepositioner 200 may determine an adjusted measure of bias of thepositioner 200 as described above, and the positioner 200 may update thedefault or nominal bias according to the adjusted measure of bias. Thus,the positioner 200 may refine a default bias or other currently usedbias at suitable times and/or over time to compensate for changes in thebias due to temperature, wear, aging of components, etc.

FIG. 5 illustrates an example control loop 500 utilizing a measure ofI/P bias, which measure of I/P bias may be generated according to themethod 400. The controller and/or positioner 200 may implement at leasta portion of the control loop 500. Specifically, the positioner 200 mayimplement a portion 504 of the control loop 500. In otherimplementations, the functionality of the portion 504 of the controlloop 500 may be divided in any suitable manner between the positioner200 and the controller.

The positioner 200 may receive pressure feedback values indicative ofpressures in or supplied to an actuator 506, such as the actuatorportion of actuator/valve assembly 202 in FIG. 3. The positioner 200 mayalso generate a control signal (e.g., a 0-1.42 mA control signal)indicative a pressure based on the pressure feedback values, a pressureset point (or “SP”), and various terms of the control loop 500 scheme.At least some of these various terms (“Ki/s,” “K,” etc.) may be added orotherwise combined with the pressure set point to generate the controlsignal, and, in particular, a measure of I/P bias may be added to adefault bias to account for a bias of the positioner 200.

Upon receiving the control signal, an I/P component 510 and arelay/spool valve component 512 of the positioner 200 may cause apressure to be supplied to the actuator 506 to produce a travel. Becausethe positioner 200 accounts for the I/P bias of the positioner 200, thepositioner 200 may precisely control a pressure supplied to the actuator506, at least within pre-defined tolerances. Such precision may be ofimportance when controlling high gain actuator/valve assemblies, becausesmall changes in pressure may result in large travels of the high gainactuator/valve assemblies. This precision may also be of importance inother types of actuator/valve assemblies to calibrate actuator/valveassemblies, or devices such as positioners coupled to actuator/valveassemblies, while the actuator/valve assemblies are in service. Further,by adjusting a bias when a valve is at a hard stop (e.g., during endpoint pressure control scenarios), some implementations of positionersmay adjust biases to account for temperature changes, wear, and aging ofcomponents without having to disturb a process (e.g., without having toshut down a particular line).

Testing Actuator/Valve Assemblies

In some implementations, controllers, such as the process controller 11,may trigger positioners, such as the positioner 200, to testactuator/valve assemblies. These tests, which may be required by certainregulatory bodies, may ensure that the actuator/valve assemblies areable to function (e.g., that an actuator or piston is able to travel).In particular, positioners, such as valve positioners coupled to ESVs,may perform partial stroke testing of actuator/valve assemblies to testan operation to open or close a valve without fully opening or closingthe valve, so as to not disrupt a process.

When performing partial stroke testing, positioners and/or controllersof the present disclosure may utilize pressure control techniques, asopposed to travel or position control techniques. In this manner,difficulties arising from loose connections, significant lost motion,high seal friction, and stick-slip dynamics may be substantiallyminimized (e.g., by reducing errors below a tolerance) and/or tests maybe performed even when travel control functionality of device is notoperational or is malfunctioning. To illustrate these points andcontrast the current pressure control techniques for partial stroketests, FIGS. 6A, 6B, and 6C illustrate a partial stroke test of apneumatic valve actuator using travel control techniques. Although aspecific pneumatic actuator exhibiting certain characteristics isdiscussed with reference to FIGS. 6A, 6B, and 6C, positioners mayutilize pressure control to test any suitable types of valves, such asESVs, compressor anti-surge valves, vent valves, etc.

In particular, FIG. 6A illustrates a plot of relative travel vs. timefor a twenty second scan of the pneumatic valve actuator, or a travelset point ramp of 1%/second to 20% displacement. The plot illustrates aninitial transition of the pneumatic valve actuator off of a hard stop,where, during this initial transition, is unloaded from a suppliedpressure to an high end pressure of a bench set (or upper bench set).These dynamics are further illustrated in the pressure vs. time graphillustrated in FIG. 6B (corresponding to the same twenty second scan).The pressure exhibits dramatic shifts during the initial transition ofthe pneumatic valve actuator off of the hard stop.

Upon examining a parametric plot of the pressure vs. relative travel forthe twenty second scan, as illustrated in FIG. 6C, one can see a clearnon-symmetric (e.g., varying in time) behavior of the pneumatic valveactuator during the partial stroke test. Bernoulli and/or choked floweffect around the pressure sensor may cause this example behavior. Thatis, high velocities within the actuator may distort reading of thepressure sensor such that the reading does not accurately reflect actualpressures within the actuator. Because certain alerts (e.g., alertscorresponding to stuck valves) may be triggered off of a pressurethreshold, distortions in pressure reading may result in false alerts.Generally, this type of behavior and/or other types of behavioroccurring during travel control (e.g., resulting from stick and slipdynamics) may complicate the partial stroke test and may cause thepressure within the pneumatic valve actuator to go open-loop (e.g., outof the control of the controller) in the event of a stuck valve.

In contrast to partial stroke tests facilitated by travel control, thecurrent techniques may utilize pressure control to perform partialstroke tests. In particular, a positioner may cause a pressure in aactuator/valve assembly to ramp from an initial pressure towards aminimum or maximum pressure value. When the minimum or maximum pressureis reached or when a travel of the actuator/valve assembly is detected,the positioner may cause the pressure to ramp back towards the initialpressure. In this manner, the pressure within the actuator/valveassembly is always under control.

FIGS. 7A, 7B, and 7C illustrate a scenario in which a partial stroketest is performed using pressure control techniques. FIGS. 7A and 7Billustrate plots of relative travel vs. time and pressure (and pressureset point) vs. time, respectively, for a 1%/second ramping of a pressurewithin a pneumatic valve actuator similar to the pneumatic valveactuator tested in FIGS. 6A, 6B, and 6C. As can be seen in FIGS. 7A and7B, the relative travel of the pneumatic valve actuator remains nearlyconstant while the pressure in the pneumatic valve actuator is rampedtowards a pressure limit (e.g., twenty pounds per square inch gauge(psig), as illustrated by the dotted line in FIG. 7B).

At a certain time (around sixty-five seconds), the pneumatic valveactuator travels, and, at this time, the ramping of the pressure may bereversed back towards the initial pressure (before reaching the pressurelimit, in this scenario). During the partial stroke test, even at timeswhen the travel of the pneumatic valve actuator remained near constant,the pressure within the pneumatic valve actuator is under control, asfurther illustrated in FIG. 7C by a symmetric and smooth pressure andtravel response of the pneumatic valve actuator.

FIG. 8 is a flow diagram of an example method 800 for testing anactuator/valve assembly, such as the actuator/valve assembly 202 in FIG.3, with pressure control techniques. The method 800 may be implementedby a suitable combination of controllers and positioners, such asprocess controller 11 and positioner 200. For ease of discussion, thecomponents of the example positioner 200, such as the partial stroketest routine 220, may be referenced in the description of the method800, but, generally, the method 800 may be utilized by any suitablecontroller or positioner to test any suitable actuator/valve assembly.

The process controller 11 and/or positioner 200 may execute the partialstroke test routine 220 to determine a pressure limit and/or travellimit (block 802). The partial stroke test routine 220 may utilize thepressure and travel limits during a controlled ramping of a pressurewithin the actuator/valve assembly 202. In some implementations, thepartial stroke test routine 220 determines the pressure limit to be apre-configured pressure value programmed, or otherwise configured, inthe partial stroke test routine 220. In other implementations, thepartial stroke test routine 220 may retrieve the pressure limit from adata storage device (e.g., database) operatively connected to theprocess controller 11, or the partial stroke test routine 220 may evendetermine the pressure limit in near real-time (e.g., when executing toperform a partial stroke test) based on user input into the processcontroller 11, current or historical pressure and/or travel feedbackvalues, etc.

The pressure limit (e.g., programmed as a parameter in the partialstroke test routine 220) may define a pressure such that theactuator/valve assembly 202 is expected to move (e.g., based on priorbench tests) as the partial stroke test routine 220 ramps a pressurewithin the actuator/valve assembly 202 to the pressure limit. In somecases, the pressure limit defines a pressure such that theactuator/valve assembly 202 does not move past a maximum travel orrelative travel (e.g., 20%) when a pressure within the actuator/valveassembly 202 is ramped to the pressure limit. In this manner, thepartial stroke test routine 220 may test the operation of theactuator/valve assembly 202 while preventing disruption of a process,which disruption may occur when the actuator/valve assembly 202 travelspast the maximum travel.

The pressure limit may be an upper pressure limit or a lower pressurelimit depending on the configuration of the actuator/valve assembly 202.For example, if the actuator/valve assembly 202 is a normally open ESV,the positioner 200 may utilize a lower pressure limit, whereas thepositioner 200 may utilize an upper pressure limit for a normally closedESV.

The partial stroke test routine 220 ramps a pressure within theactuator/valve assembly 202 from an initial pressure within theactuator/valve assembly 202 towards the pressure limit (block 804). Forexample, the partial stroke test routine 220 and/or other components ofthe control logic 208 may implement at least portions of a pressurecontrol loop, such as one of the pressure control loops discussed withreference to FIGS. 5 and 11, to control the pressure to ramp towards thepressure limit.

The partial stroke test routine 220 may then determine if theactuator/valve assembly 202 has reached the travel limit (block 806).For example, one or more sensors sensing travel of the actuator/valveassembly 202 may feedback data indicative of a travel or relative travel(e.g., percentage of total travel) to the positioner 200. In someimplementations, the positioner 200 may continue to ramp the pressureuntil a certain percentage of total travel of the actuator/valveassembly 202 is detected (e.g., a 20% relative travel limit), whereas,in other implementations, the positioner 200 may continue to ramp thepressure until any amount (e.g., any finite amount) of travel of theactuator/valve assembly 202 is detected.

If the travel limit of the actuator/valve assembly 202 is reached, theflow may continue to block 808, where the partial stroke test routine220 may reverse the ramping of the pressure such that the pressure isramped back towards the initial pressure. However, if no travel, or arelative travel less than the travel limit, is detected, the flow maycontinue to block 810. At block 810, the partial stroke test routine 220may determine if the pressure limit has been reached. If the pressurelimit has been reached, the flow may continue to block 808, but, if thepressure limit has not been reached, the flow may revert to block 804where the ramping of the pressure continues towards the pressure limit.

In some implementations, instead of simply reversing the ramping of thepressure upon a detection of travel or a detection of a relative amountof travel, the positioner 200 (e.g., the partial stroke test routine220) may control the pressure in the actuator/valve assembly 202 to: (i)step back towards the initial pressure by a finite amount, and (ii) thencontinue ramp back towards the initial pressure. That is, the positioner200 may near instantaneously reinitialize the pressure before rampingthe pressure back towards the initial pressure. In this manner, thepositioner 200 may minimize further drifting of the actuator/valveassembly 202 past the detected travel or amount of relative travel.

FIGS. 9A, 9B, and 9C include plots similar to those of FIGS. 7A, 7B, and7B illustrating pressure (and pressure set point) vs. time, relativetravel vs. time, and pressure vs. relative travel for a ramping of apressure within a pneumatic valve actuator at a rate of 1%/second.However, instead of simply reversing the ramping of pressure asillustrated in FIG. 7A, FIG. 9A illustrates a stepping of pressure upondetecting travel of the pneumatic valve actuator (at approximatelysixty-five seconds) and a subsequent ramping of the pressure back to theinitial pressure. By employing this stepping of the pressure, apositioner may prevent drifting of a pneumatic valve actuator past amaximum desired travel (illustrated by the dotted line in FIG. 9B).

Although FIGS. 9A, 9B, and 9C include curves illustrating a stepping ofa pressure upon detecting travel of a pneumatic valve actuator,controllers and/or positioner may step pressures at any suitable timesduring a partial stroke test. For example, the partial stroke testroutine 220 may: (i) step a pressure at the beginning of a partialstroke test from an initial pressure to a pre-defined pressure, and (ii)then ramp from the pre-defined pressure towards a pressure limit. Such aprocedure may allow more time efficient partial stroke tests. Generally,a stepping of pressure may occur at the beginning, towards the end, upona detection of travel, and/or at any other point during a partial stroketest.

Moreover, a positioner may further reduce a drifting of a pneumaticvalve actuator past a maximum travel or relative travel by employingdynamic ramp rates, as illustrated in FIGS. 10A, 10B, and 10C. FIGS.10A, 10B, and 10C include plots similar to those of FIGS. 7A, 7B, 7B,9A, 9B, and 9C illustrating pressure (and pressure set point) vs. time,relative travel vs. time, and pressure vs. relative travel for a rampingof a pressure within a pneumatic valve actuator. However, in contrast toFIGS. 7A, 7B, 7B, 9A, 9B, and 9C, in FIGS. 10A, 10B, and 10C, two ramprates are utilized in the ramping of the pressure toward the pressurelimit. For example, a ramping of the pressure may be slowed as thepressure approaches the pressure limit at one or more thresholds ofpressure. As can be seen in FIG. 10B, such a dynamic ramping furtherprevents drifting a pneumatic valve actuator past a maximum desiredtravel or relative travel (illustrated by the dotted line in FIG. 10B).Although two ramp rates are utilized in the test depicted in FIGS. 10A,10B, and 10C, it is understood that any number of ramp rates may beutilized in ramping a pressure towards a pressure limit and/or inreversing the ramping of the pressure back towards an initial pressure.

FIG. 11 illustrates an example control loop 1100 which may be utilized(e.g., by the positioner 200) to perform partial stroke or other testswith pressure control techniques, as described further with reference toFIG. 8. The process controller 11 and/or positioner 200 may implement atleast a portion of the control loop 1100. Specifically, the examplepositioner 200 may implement a portion 1104 of the control loop 1100. Aswith the control loop 500, some implementations of the control loop 1100may include components of the control loop 1100 (e.g., of the portion1104) distributed in any suitable manner between a positioner and acontroller, such as the process controller 11.

In the control loop 1100, the positioner 200 may receive pressurefeedback values from an actuator 1106, such as the actuator portion ofactuator/valve assembly 202 in FIG. 3. However, in the control loop1100, the positioner 200 may also receive travel feedback values fromthe actuator 1106. The positioner 200 may generate a control signal(e.g., 1 mA nominal signal plus or minus 0.4 mA) indicative a pressurebased on an internal pressure control portion 1108 of the control loop1100 nested within an outer travel control portion 1110 of the controlloop 1100.

The internal pressure control portion 1108 of the control loop 1100 maybe substantially similar to portions of the control loop 500 configuredto generate a control signal for the positioner 200 based on pressurefeedback values, a pressure set point (or “SP”), and various integral,proportional, or derivative terms of the internal pressure controlportion 1108. By nesting this internal pressure control portion 1108inside the outer travel control portion 1110 of the control loop 1100,the positioner 200 implementing the control loop 1100 may ramp pressureswithin the actuator/valve assembly 202 until certain specific travelconditions are met. For example, the positioner 200 may ramp pressureswithin the actuator/valve assembly 202 until the actuator/valve assembly202 has a traveled any finite amount, a certain pre-defined amount, acertain percentage of total travel, etc., as controlled by a travelcontroller 1112 of the outer travel control portion of the control loop1100.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, it will be apparent to those of ordinaryskill in the art that changes, additions or deletions may be made to thedisclosed embodiments without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method of testing a valve actuator coupled to aprocess control device, the method comprising: controlling, with theprocess control device, a pressure within the valve actuator to rampfrom an initial pressure towards a pre-determined pressure limit;monitoring a position of the valve actuator to detect a travel of thevalve actuator; upon one of the pressure within the valve actuatorreaching the pre-determined pressure limit or detecting the travel ofthe valve actuator, controlling the pressure within the valve actuatorto step to a stepped pressure higher or lower than the pre-determinedpressure limit or the pressure at which the travel of the valve actuatoris detected; and after controlling the pressure within the valveactuator to step to the stepped pressure, controlling the pressurewithin the valve actuator to return ramp from the stepped pressure tothe initial pressure.
 2. The method of claim 1, further comprising:determining a bias of the process control device by comparing a measuredpressure within the valve actuator to a set point pressure utilized bythe process control device; wherein controlling the pressure within thevalve actuator to ramp from the initial pressure towards thepre-determined pressure limit includes accounting for the bias of theprocess control device.
 3. The method of claim 1, wherein monitoring theposition of the valve actuator to detect the travel of the valveactuator includes monitoring the position of the valve actuator todetect a predetermined amount of travel corresponding to a percentage ofa possible travel of the valve actuator.
 4. The method of claim 3,wherein: controlling the pressure to ramp from the initial pressuretowards the pre-determined pressure limit includes controlling thepressure based on a pressure control loop implemented by the processcontrol device; monitoring the position of the valve actuator to detectthe certain amount of travel of the valve actuator includes monitoringthe position of the valve actuator based on a travel control loopimplemented by the process control device; and the travel control loopincludes the pressure control loop.
 5. The method of claim 1, whereinmonitoring the position of the valve actuator to detect the travel ofthe valve actuator includes monitoring the position of the valveactuator to detect any non-zero travel of the valve actuator.
 6. Themethod of claim 5, wherein controlling the pressure to ramp from theinitial pressure towards the pre-determined pressure limit andmonitoring the position of the valve actuator includes controlling thepressure and monitoring the position of the valve actuator based on apressure control loop separate from any other control loops.
 7. Themethod of claim 1, wherein the valve actuator is part of a safetyshutdown system.
 8. The method of claim 1, wherein the valve actuator iscoupled to a spool valve or a relay valve.
 9. A system, comprising: anactuator; and a process control device coupled to the actuator, whereinthe process control device is configured to: control a pressure withinthe actuator to ramp from an initial pressure towards a pre-determinedpressure limit; gather measured values indicative of a position of theactuator to detect a travel of the actuator; upon one of the pressurewithin the actuator reaching the pre-determined pressure limit ordetecting the travel of the actuator, control the pressure within theactuator to step to a stepped pressure higher or lower than thepre-determined pressure limit or the pressure at which the travel of theactuator is detected; and after controlling the pressure within thevalve actuator to step to the stepped pressure, control the pressurewithin the actuator to ramp from the stepped pressure to the initialpressure.
 10. The system of claim 9, wherein the process control deviceis further configured to: determine a bias of the process control deviceby comparing a measured pressure within the actuator to a set pointpressure utilized by the process control device; wherein controlling thepressure within the actuator to ramp from the initial pressure towards apre-determined pressure limit includes accounting for the bias of theprocess control device.
 11. The system of claim 10, wherein determiningthe bias of the process control device includes: determining acalibration pressure value corresponding to a particular state of theactuator; controlling, with the process control device, the pressurewithin the actuator according to the set point pressure, wherein the setpoint pressure is based on the calibration pressure value such that theactuator maintains the particular state; measuring an actual pressurewithin the actuator; and comparing the actual pressure to the set pointpressure to determine the bias of the process control device.
 12. Thesystem of claim 9, wherein the process control device is furtherconfigured to determine the pre-determined pressure limit by driving theactuator to a hard stop and measuring a pressure corresponding to thehard stop.
 13. A system, comprising: an actuator; and a process controldevice coupled to the actuator, wherein the process control device isconfigured to: ramp a pressure within the actuator from an initialpressure towards a pressure threshold according to a first ramp rate;and upon reaching the pressure threshold, ramp the pressure within theactuator from the pressure threshold to a pre-determined pressure limitaccording to a second ramp rate different from the first ramp rate;gather measured values indicative of a position of the actuator todetect a travel of the actuator; and upon one of the pressure within theactuator reaching the pre-determined pressure limit or detecting thetravel of the actuator, control the pressure within the actuator toreturn to the initial pressure.
 14. The system of claim 13, whereincontrolling the pressure within the actuator to ramp from the initialpressure towards the pre-determined pressure limit includes controllingthe pressure within the actuator to ramp from the initial pressuretowards the pre-determined pressure limit according to three or moreramp rates.
 15. The system of claim 13, wherein controlling the pressurewithin the actuator to return to the initial pressure includes:controlling the pressure in the actuator to step from one of thepre-determined pressure limit or the pressure at which the travel of theactuator is detected to a stepped pressure higher or lower than thepre-determined pressure limit or the pressure at which the travel of theactuator is detected; and after controlling the pressure to step,controlling the pressure to ramp from the stepped pressure towards theinitial pressure.
 16. A computer device, comprising: one or moreprocessors; and one or more non-transitory memories having computerexecutable instructions stored thereon that, when executed by the one ormore processors, cause the computer device to: control a pressure withinan actuator to ramp from an initial pressure towards a pre-determinedpressure limit; gather measured values indicative of a position of theactuator to detect a travel of the actuator; upon one of the pressurewithin the actuator reaching the pre-determined pressure limit ordetecting the travel of the actuator, controlling the pressure to stepto a stepped pressure higher or lower than the pre-determined pressurelimit or the pressure at which the travel of the actuator is detected;and after controlling the pressure to step, control the pressure withinthe actuator to ramp from the stepped pressure towards the initialpressure to return to the initial pressure.
 17. A computer device,comprising: one or more processors; and one or more non-transitorymemories having computer executable instructions stored thereon that,when executed by the one or more processors, cause the computer deviceto: control a pressure within an actuator to ramp from an initialpressure towards a pre-determined pressure limit according to two ormore ramp rates; gather measured values indicative of a position of theactuator to detect a travel of the actuator; and upon one of thepressure within the actuator reaching the pre-determined pressure limitor detecting the travel of the actuator, control the pressure within theactuator to return to the initial pressure.
 18. A method of testing avalve actuator coupled to a process control device, the methodcomprising: controlling, with the process control device, a pressurewithin the valve actuator to ramp from an initial pressure towards apre-determined pressure limit by ramping the pressure from the initialpressure towards a pressure threshold according to a first ramp rateand, upon reaching the pressure threshold, ramping the pressure from thepressure threshold to the pre-determined pressure limit according to asecond ramp rate, different from the first ramp rate; monitoring aposition of the valve actuator to detect a travel of the valve actuator;and upon one of the pressure within the valve actuator reaching thepre-determined pressure limit or detecting the travel of the valveactuator, controlling the pressure within the valve actuator to returnto the initial pressure.